Production of polycrystalline silicon in substantially closed-loop processes that involve disproportionation operations

ABSTRACT

Production of polycrystalline silicon in substantially closed-loop processes and systems is disclosed. The processes and systems generally involve disproportionation of trichlorosilane to produce silane or dichlorosilane and thermal decomposition of silane or dichlorosilane to produce polycrystalline silicon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/328,029, filed Dec. 16, 2011, which claims the benefit of U.S.Provisional Application No. 61/425,069, filed Dec. 20, 2010, both ofwhich are incorporate herein by reference.

FIELD OF THE DISCLOSURE

The field of the present disclosure relates to production ofpolycrystalline silicon in substantially closed-loop processes and,particularly, processes that involve disproportionation oftrichlorosilane produced from metallurgical grade silicon.

BACKGROUND

Polycrystalline silicon is a vital raw material used to produce manycommercial products including, for example, integrated circuits andphotovoltaic (i.e., solar) cells. Polycrystalline silicon is oftenproduced by a chemical vapor deposition mechanism in which silicon isdeposited from a thermally decomposable silicon compound onto siliconparticles in a fluidized bed reactor or onto silicon rods as in aSiemens-type reactor. The seed particles continuously grow in size untilthey exit the reactor as polycrystalline silicon product (i.e.,“granular” polycrystalline silicon). Suitable decomposable siliconcompounds include, for example, silane and halosilanes such astrichlorosilane.

Silane may be produced by reacting silicon tetrafluoride with an alkalior alkaline earth metal aluminum hydride such as sodium aluminumtetrahydride as disclosed in U.S. Pat. No. 4,632,816, which isincorporated herein by reference for all relevant and consistentpurposes. Silane may alternatively be produced by the so-called “UnionCarbide Process” in which metallurgical-grade silicon is reacted withhydrogen and silicon tetrachloride to produce trichlorosilane asdescribed by Müller et al. in “Development and Economic Evaluation of aReactive Distillation Process for Silane Production,” Distillation andAdsorption: Integrated Processes, 2002, which is incorporated herein byreference for all relevant and consistent purposes. The trichlorosilaneis subsequently taken through a series of disproportionation anddistillation steps to produce a silane end-product. The startingcompounds of silane production are relatively expensive components insilane-based production of polycrystalline silicon.

A continuing need exists for processes for producing polycrystallinesilicon that reduce the amount of hydrogen and chlorine used relative toconventional methods and for methods that are capable of producingpolycrystalline silicon in a substantially closed-loop process relativeto hydrogen or chlorine (e.g., hydrogen chloride). A continuing needalso exists for systems for producing polycrystalline silicon that makeuse of such processes.

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the disclosure, which aredescribed and/or claimed below. This discussion is believed to behelpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentdisclosure. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

SUMMARY

One aspect of the present disclosure is directed to a substantiallyclosed-loop process for producing polycrystalline silicon.Trichlorosilane is introduced into a disproportionation system toproduce silicon tetrachloride and at least one of silane anddichlorosilane. Silane or dichlorosilane produced from thedisproportionation system is introduced into a fluidized bed reactor toproduce polycrystalline silicon and an effluent gas containing hydrogenand unreacted silane or dichlorosilane. An amount of silicontetrachloride produced from the disproportionation system and an amountof hydrogen from the effluent gas are introduced into a hydrogenationreactor to produce trichlorosilane and hydrogen chloride. An amount ofhydrogen chloride produced from the hydrogenation reactor and siliconare introduced into a chlorination reactor to produce a chlorinated gascontaining trichlorosilane and silicon tetrachloride. Trichlorosilaneproduced from the chlorination reactor is introduced into thedisproportionation system to produce silicon tetrachloride and at leastone of silane and dichlorosilane.

Various refinements exist of the features noted in relation to theabove-mentioned aspects of the present disclosure. Further features mayalso be incorporated in the above-mentioned aspects of the presentdisclosure as well. These refinements and additional features may existindividually or in any combination. For instance, various featuresdiscussed below in relation to any of the illustrated embodiments of thepresent disclosure may be incorporated into any of the above-describedaspects of the present disclosure, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a system for producing polycrystallinesilicon by the thermal decomposition of silane or dichlorosilane;

FIG. 2 is a flow diagram of a disproportionation system for convertingtrichlorosilane to silane; and

FIG. 3 is a flow diagram of a separation system for separatingchlorosilanes, hydrogen and hydrogen chloride.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION

In accordance with the present disclosure, substantially closed-loopprocesses and systems for producing polycrystalline silicon from silaneare provided. As used herein, the phrases “substantially closed-loopprocess” or “substantially closed-loop system” refer to a process orsystem in which the compound with respect to which the system or processis substantially closed-loop is not withdrawn from the system or processother than as an impurity and is not fed into the system or processother than as in a make-up stream. As used herein, the systems andprocesses are substantially closed-loop with respect to all compoundsother than silicon such as, for example, trichlorosilane, silicontetrachloride, silane, hydrogen chloride and/or hydrogen gas.

Closed-Loop Processes for Producing Polycrystalline Silicon

In several embodiments of the present disclosure and as shown in FIG. 1,a source of silicon 3 and hydrogen chloride 6 are introduced andcontacted in a chlorination reactor 7 to produce a chlorinated gas 10.The chlorinated gas 10 contains trichlorosilane and silicontetrachloride as well as hydrogen and unreacted hydrogen chloride.Trichlorosilane and silicon tetrachloride may be produced in thechlorination reactor 7 according to the following reactions,Si+3HCl→SiHCl₃+H₂  (1),SiHCl₃+HCl→SiCl₄+H₂  (2).

In this regard it should be understood that, as used herein, “contact”of two or more reactive compounds generally results in a reaction of thecomponents and the terms “contacting” and “reacting” are synonymous asare derivations of these terms and these terms and their derivationsshould not be considered in a limiting sense. Typically the source ofsilicon 3 is metallurgical grade silicon; however, it should beunderstood that other sources of silicon may be used such as, forexample, sand (i.e., SiO₂), quartz, flint, diatomite, mineral silicates,fused silica, fluorosilicates and mixtures thereof. The particle size ofthe silicon may range from about 10 μm to about 750 μm or from about 50μm to about 250 μm prior to introduction into the reactor 7. Increasingthe particle size generally decreases the reaction rate while smallersizes result in more particles being entrained in spent reactor gasesand difficulty in fluidization as a result of increased cohesive forcesamong the smaller diameter particles.

The chlorination reactor 7 may be a fluidized bed reactor in whichsilicon 3 is suspended in the incoming hydrogen chloride gas 6. Thereactor 7 may be operated at a temperature of at least about 250° C.and, in other embodiments, at least about 300° C. (e.g., from about 250°C. to about 450° C. or from about 300° C. to about 400° C.). In view ofthe exothermic nature of reactions (1) and (2), the chlorination reactor7 may include cooling means (e.g., cooling coils in thermalcommunication with the reactor bed or a cooling jacket) to assist incontrolling the temperature of the reactor. In this regard, it should beunderstood that while the chlorination reactor 7 may be a fluidized bedreactor, other reactor designs may be used without limitation.

The reactor 7 may be operated at a pressure (i.e., overhead gaspressure) of at least about 1 bar such as, for example, from about 1 barto about 10 bar, from about 1 bar to about 7 bar or from about 2 bar toabout 5 bar. The incoming hydrogen chloride stream 6 may include anamount of impurities such as chlorosilanes (e.g., silicon tetrachlorideand/or trichlorosilane). In various embodiments of the presentdisclosure, the hydrogen chloride stream 6 comprises at least about 80vol % hydrogen chloride, at least about 90 vol %, at least about 95 vol% or even at least about 99 vol % hydrogen chloride (e.g., from about 80vol % to about 99 vol % or from about 90 vol % to about 99 vol %).

The chlorination reactor 7 may include an amount of catalyst to promoteformation of trichlorosilane relative to formation of silicontetrachloride in the chlorinated gas 10. For instance, the chlorinationreactor 7 may include a group VIII metal catalyst (e.g., iron, cobalt,nickel, vanadium and/or platinum) or a catalyst containing aluminum,copper or titanium metal as disclosed in U.S. Pat. No. 5,871,705, whichis incorporated herein by reference for all relevant and consistentpurposes. The reactor 7 may also include an amount of one or more alkalimetal compounds (e.g., lithium chloride, sodium chloride, potassiumchloride, cesium chloride, rubidium chloride, sodium sulfate and/orsodium nitrate) to increase the selectivity toward trichlorosilane. Thereactor 7 may be operated at from about 1.1 times to about 8 times theminimum fluidization velocity or from about 1.5 to about 4 times theminimum fluidization velocity.

The conversion of hydrogen chloride in the chlorination reactor 7 mayvary depending on the reaction conditions and, typically, will be atleast about 50%, at least about 65%, at least about 80%, at least about90% and in some embodiments, conversion may approach 100% (e.g., fromabout 50% to about 100% or from about 80% to about 100%). Selectivitytoward trichlorosilane may be at least about 50%, at least about 65% oreven at least about 80% (e.g., from about 50% to about 90% or from about70% to about 90%).

The chlorinated gas 10 is introduced into a separation system 4 toseparate trichlorosilane and silicon tetrachloride (designatedcollectively as 26) from hydrogen 22 and hydrogen chloride 6. Unreactedhydrogen chloride 6 is recycled for use in the chlorination reactor 7.Hydrogen 22 is sent forward to a hydrogenation reactor 60 discussedfurther below. The trichlorosilane and silicon tetrachloride 26 areintroduced into a disproportionation system 76 in which silicontetrachloride 57 is separated out and silane 29 is produced. Optionally,the trichlorosilane and silicon tetrachloride 26 may be introduced intoa separator (not shown) such as a stripper column to separate one ormore impurities from the gases and, for example, to separate light-endimpurities (i.e., compounds with a boiling point less than silane) priorto introduction into the disproportionation system. Such strippercolumns may be operated at a pressure of at least about 3 bar (e.g.,from about 3 bar to about 10 bar).

An exemplary separation system 4 for use in the processes of the presentdisclosure is shown in FIG. 3. The separation system 4 includes achlorosilane separator 40 to separate trichlorosilane and silicontetrachloride 26 from hydrogen and hydrogen chloride 42. Thechlorosilane separator 40 may be constructed according to any of themethods for separating gaseous components as appreciated by those ofskill in the art. In some embodiments, the separator 40 is avapor-liquid separator. Examples of such vapor-liquid separators includevessels in which the pressure and/or temperature of the incoming gas(e.g., chlorinated gas 10 and hydrogenated gas 63 described below) isreduced causing the higher boiling-point gases (e.g., silicontetrachloride and trichlorosilane) to condense and separate from lowerboiling point gases (e.g., hydrogen and hydrogen chloride). Suitablevessels include vessels which are commonly referred to in the art as“knock-out drums.” Optionally, the vessel may be cooled to promoteseparation of gases. Alternatively, the separator 40 may be one or moredistillation columns.

Hydrogen and hydrogen chloride 42 are introduced into a hydrogenseparator 47 to produce a hydrogen chloride gas 6 that is introducedinto the chlorination reactor 7 and a hydrogen gas 22 that is introducedinto a hydrogenation reactor 60. The hydrogen separator 47 may be anytype of separator suitable to separate hydrogen from hydrogen chloride.An exemplary separator 47 is a bubbler in which hydrogen and hydrogenchloride are bubbled through a vessel containing a fluid (e.g., water)and, typically, in which the fluid is continuously introduced (notshown) and removed. Hydrogen chloride is adsorbed within the fluid(e.g., water) while separated hydrogen is removed from the vessel as agas. Alternatively, the hydrogen separator 47 may be a vapor-liquidseparator (e.g., knock-out drum) and the hydrogen and hydrogen chloride42 may be partially condensed prior to introduction into the separator47. In alternative embodiments, the hydrogen separator 47 includes useof one or more distillation columns to separate hydrogen 22 fromhydrogen chloride 6. In this regard, it should be understood thatmethods and apparatus for separating and purifying hydrogen and hydrogenchloride other than those recited above may be used in any combination(e.g., in series or in parallel) without departing from the scope of thepresent disclosure.

The disproportionation system 76 to which silicon tetrachloride andtrichlorosilane are introduced from the separation system may includeany unit operations customary in disproportionation operations asappreciated by those of skill in the art, and particularly, equipmentsuitable for conversion of trichlorosilane to silane such as disclosedin U.S. Pat. No. 4,676,967 which is incorporated herein by reference forall relevant and consistent purposes. An exemplary disproportionationsystem 76 for producing silane 29 is shown in FIG. 2. Thedisproportionation system 76 includes a first distillation column 65, asecond distillation column 67, a third distillation column 56, a firstdisproportionation reactor 50 and a second disproportionation reactor52. Silicon tetrachloride and trichlorosilane 26 are introduced into thefirst distillation column 65 to separate silicon tetrachloride into abottoms fraction 57 and to separate trichlorosilane into an overheadfraction 69. Dichlorosilane and silicon tetrachloride 9 produced fromthe first disproportionation reactor 50 described below is alsointroduced into the first distillation column 65 to separatedichlorosilane into the overhead fraction 69 and silicon tetrachlorideinto the bottoms fraction 57. The first distillation column 65 may beoperated at a pressure of at least about 2 bar (e.g., from about 2 barto about 5 bar) and at an overhead temperature of at least about −25°C., at least about 25° C. or at least about 75° C. (e.g., from about−25° C. to about 150° C. or from about 0° C. to about 75° C.).

The trichlorosilane-containing overhead fraction 69 produced from thefirst distillation column 65 is introduced into the second distillationcolumn 67 to separate trichlorosilane into a bottoms fraction 5 anddichlorosilane into an overhead fraction 15. The second distillationcolumn 67 may be operated at a pressure of at least about 10 bar (e.g.,from about 10 bar to about 35 bar or from about 20 bar to about 25 bar)and at an overhead temperature of at least about −75° C., at least about−50° C. or at least about −25° C. (e.g., from about −75° C. to about100° C. or from about −50° C. to about 50° C.).

The trichlorosilane-containing bottoms fraction 5 produced from thesecond distillation column 67 is introduced into the firstdisproportionation reactor 50 to produce a first disproportionationreactor product gas 9 that contains dichlorosilane and silicontetrachloride according to the following reaction,2SiHCl₃→SiH₂Cl₂+SiCl₄  (3).

The reactor 50 may include one or more catalysts therein to promotereaction (3) including, for example, polymeric resins (e.g., AMBERLYSTA21).

The first disproportionation reactor product gas 9 is introduced intothe first distillation column 65. The dichlorosilane-containing overheadfraction 15 produced from the second distillation column 67 isintroduced into the second disproportionation reactor 52 to produce asecond disproportionation reactor product gas 98 containingtrichlorosilane and silane according to the reactions shown below,2SiH₂Cl₂→SiH₃Cl+SiHCl₃  (4),2SiH₃Cl→SiH₂Cl₂+SiH₄  (5).The net conversion to silane and trichlorosilane (i.e., the sum ofreactions (4) and (5)) is shown by the following reaction,3SiH₂Cl₂→2SiHCl₃+SiH₄  (6).In this regard it should be understood that reactions (3)-(6) do notrepresent the entire set of reactions that may occur in thedisproportionation system 76 and other reactions may occur resulting inproduction of other intermediates and by-products within the system 76including, for example, monochlorosilane, trichlorosilane and/or silane.The reactor 52 may include one or more catalysts therein to promote thereaction including, for example, polymeric resins (e.g., AMBERLYST A21).

The second disproportionation reactor product gas 98 is introduced intothe third distillation column 56 to separate silane into an overheadfraction 29 and trichlorosilane into a bottoms fraction 94. The thirddistillation column 56 may be operated at a pressure of at least about10 bar (e.g., from about 10 bar to about 35 bar or from about 20 bar toabout 25 bar) and at an overhead temperature of at least about −75° C.,at least about −50° C. or at least about −25° C. (e.g., from about −75°C. to about 100° C. or from about −50° C. to about 50° C.). Silane 29 isvaporized and introduced into the fluidized bed reactor 30 (FIG. 1) forproduction of polycrystalline silicon 27. The trichlorosilane-containingbottoms fraction 94 is introduced into the second distillation column67. In this regard, it should by understood that systems and processesfor producing silane other than as shown in FIG. 2 may be used withoutlimitation including systems and processes wherein the reactors and/orcolumns shown therein are rearranged, added or eliminated.

It should be understood that while the substantially closed-loopprocesses and systems described herein are generally described withreference to production and thermal decomposition of silane, thedisproportionation system 76 may be modified to produce dichlorosilanerather than silane. For example, the system 76 shown in FIG. 2 mayoperate without a second disproportionation reactor and thirddistillation column 56. The dichlorosilane containing overhead fraction15 produced from the second distillation column 67 may be vaporized andintroduced into the fluidized bed reactor 30 (FIG. 1) for production ofpolycrystalline silicon 27. The fluidized bed reactor 30 to whichdichlorosilane is introduced may generally be operated in accordancewith the silane-based fluidized bed reactor 30 described below. In thisrespect, dichlorosilane may decompose to form hydrogen and/or hydrogenchloride by-product and any hydrogen may be separated and introducedinto the hydrogenation reactor 60 and any separated hydrogen chloridemay be introduced into the chlorination reactor 7.

Referring again to FIG. 1, silicon tetrachloride 57 separated in thedisproportionation system 76 is introduced into the hydrogenationreactor 60 to produce a hydrogenated gas 63 that includestrichlorosilane, hydrogen chloride, unreacted hydrogen and unreactedsilicon tetrachloride. The hydrogenated gas 63 is introduced into theseparation system 4 to separate the components thereof. Silicontetrachloride 57 that is removed from the disproportionation system 60is reacted with hydrogen 22 to produce trichlorosilane according to thefollowing reaction,SiCl₄+H₂→SiHCl₃+HCl  (7).

The hydrogenation reactor 60 may be a bubbler in which hydrogen 22 isbubbled through liquid silicon tetrachloride 57 to form trichlorosilane.Alternatively, silicon tetrachloride 57 is vaporized and the hydrogen 22and silicon tetrachloride 57 are heated and reacted in a pressurizedreaction vessel. In this regard, any vessel suitable for thehydrogenation reaction as appreciated by those of skill in the art maybe used without limitation. The contents of the reaction vessel may beheated to a temperature of at least about 800° C. to convert silicontetrachloride to trichlorosilane. In some embodiments, silicontetrachloride 57 and hydrogen 22 are heated to a temperature of at leastabout 900° C., at least about 1000° C. or even at least about 1100° C.(e.g., from about 800° C. to about 1500° C., from about 800° C. to about1200° C. or from about 1000° C. to about 1200° C.). The reaction vesselmay also be pressurized to promote formation of trichlorosilane. Forinstance, the hydrogenation reactor 60 may be operated at a pressure ofat least about 2 bar and, in other embodiments, at least about 5 bar, atleast about 10 bar or even at least about 15 bar (e.g., from about 2 barto about 20 bar or from about 8 bar to about 15 bar). The ratio ofhydrogen to silicon tetrachloride introduced into the rector 60 may varydepending on the reaction conditions. Use of a stoichiometric excess ofhydrogen typically results in increased conversion to trichlorosilane.In various embodiments, the molar ratio of hydrogen to silicontetrachloride is at least about 1:1, at least about 2:1 or even at leastabout 3:1 (e.g., from about 1:1 to about 5:1 or from about 1:1 to about3:1).

Generally, at least about 20% of silicon tetrachloride is converted totrichlorosilane in the hydrogenation reactor 60 with conversions of atleast about 30%, at least about 40% or even at least about 50% beingpossible (e.g., from about 20% to about 60% conversion). The resultinghydrogenated gas 63 contains trichlorosilane, unreacted silicontetrachloride, unreacted hydrogen and hydrogen chloride. Depending onthe amount of excess hydrogen 22 added to the reactor, the amount oftrichlorosilane in the hydrogenated gas 63 may be at least about 5 vol %and, in other embodiments, at least about 10 vol %, at least about 25vol %, or at least about 40 vol % (e.g., from about 5 vol % to about 50vol %, from about 5 vol % to about 20 vol % or from about 5 vol % toabout 10 vol %). Likewise, the amount of hydrogen chloride in thehydrogenated gas may be at least about 5 vol % and, in otherembodiments, at least about 10 vol %, at least about 25 vol %, or atleast about 40 vol % (e.g., from about 5 vol % to about 50 vol %, fromabout 5 vol % to about 20 vol % or from about 5 vol % to about 10 vol%). The amount of unreacted silicon tetrachloride may be at least about10 vol %, at least about 20 vol %, at least about 30 vol % or at leastabout 40 vol % of the hydrogenated gas stream 63 (e.g., from about 10vol % to about 50 vol %, from about 10 vol % to about 30 vol % or fromabout 15 vol % to about 25 vol %). The remainder of the hydrogenated gas63 is typically hydrogen. For instance, the hydrogenated gas 63 mayinclude at least about 40 vol % hydrogen or, as in other embodiments, atleast about 50 vol %, at least about 60 vol %, at least about 70 vol %or even at least about 80 vol % hydrogen (e.g., from about 40 vol % toabout 90 vol %, from about 50 vol % to about 80 vol % or from about 60vol % to about 80 vol %). The hydrogenated gas 63 is introduced into theseparation system 4 to separate the components thereof.

Silane 29 (or dichlorosilane as described above) produced from thedisproportionation system 76 is introduced into the fluidized bedreactor 30 in which it fluidizes growing silicon seed particles toproduce polycrystalline silicon which may be withdrawn from the reactor30 as polycrystalline silicon product 27. Polycrystalline silicon 27 isproduced from silane 29 with formation of hydrogen by-product accordingto the following pyrolysis reaction,SiH₄→Si+2H₂  (8).

Polycrystalline silicon 27 may be withdrawn from the reactor 30intermittently or continually through a product withdrawal tube and aneffluent gas 39 that includes hydrogen, unreacted silane (ordichlorosilane) and silicon dust may be withdrawn from the upper portionof the reactor 30. The effluent gas 39 may contain up to about 15 wt %silicon dust and up to about 5 wt % unreacted silane. Dust may beremoved from the effluent gas by use of a particulate separator (notshown). Suitable particulate separators include, for example, bagfilters, cyclonic separators and liquid scrubbers. Silicon dust may berecycled for use in the reactor 30 as disclosed in U.S. Pat. Pub. No.2009/0324819, which is incorporated herein by reference for all relevantand consistent purposes. Alternatively, the silicon dust may be disposedof or even collected as a product when it contains low levels of metalimpurities (e.g., when the particulate separator system includesceramic, quartz or silicon carbide surfaces). The dust-depleted effluentgas may be compressed (e.g., from about 5 bar to about 25 bar) and/orpurified and a portion 41 of the effluent gas 39 may be reintroducedinto the reactor 30 as a carrier for silane 29. The remainder 43 of theeffluent gas 43 may be introduced into the hydrogenation reactor 60.Dust-depleted effluent gas 39 may be purified by any of the methodsknown by those of skill in the art (e.g., adsorption). In severalembodiments of the present disclosure, at least a portion of theeffluent gas is introduced into the separation system 4. An amount ofhydrogen (e.g., an amount of hydrogen 22 withdrawn from the purificationsystem) may be returned to the fluidized bed reactor 30 as a carrier gasfor silane 29.

The fluidized bed reactor 30 may be operated at an overhead pressure offrom about 3 bar to about 15 bar and the incoming gases (silane 29 andrecycled effluent gas 41) may be pre-heated to a temperature of at leastabout 200° C. (e.g., from about 200° C. to about 500° C. of from about200° C. to about 350° C.). The reactor 30 may be maintained at atemperature of at least about 600° C. (e.g., 600° C. to about 900° C. orfrom about 600° C. to about 750° C.) by use of external heating meanssuch as induction heating or use of resistive heating elements. The gasvelocity through the fluidized bed reactor 30 may be generallymaintained at a velocity of from about 1 to about 8 times the minimumfluidization velocity necessary to fluidize the particles within thefluidized bed. The mean diameter of the particulate polycrystallinesilicon that is withdrawn from the reactor 30 may be at least about 600μm (e.g., from about 600 μm to about 1500 μm or from about 800 μm toabout 1200 μm). The mean diameter of the silicon seed particlesintroduced into the reactor may be less than about 600 μm (e.g., fromabout 100 μm to about 600 μm). Quench gases may be introduced into thereactor 30 (e.g., at a freeboard region of the reactor) to reduce thetemperature of the effluent gas 39 before being discharged from thereactor to suppress formation of silicon dust. The fluidized bed reactormay include an outer shell in which an inert gas is maintained at apressure above the pressure of the process gases (e.g., a differentialpressure within the range of about 0.005 bar to about 0.2 bar) to ensureprocess gases do not flow through cracks and holes within the reactionchamber. Silane may be directed to the core region of the reactor andcarrier gas (e.g., hydrogen) may be directed to the peripheral portionof the reactor near the reactor walls to reduce the deposition ofsilicon on the walls of the reactor as disclosed in U.S. Pat. Pub. No.2009/0324479 and U.S. Pat. Pub. No. 2011/0158888, both of which areincorporated herein by reference for all relevant and consistentpurposes. In some embodiments of the present disclosure, the conversionof silane in the fluidized bed reactor may be at least about 70%, atleast about 80%, at least about 90% or even at least about 95% (e.g.,from about 70% to about 99% or from about 90% to about 99%).

Hydrogen and/or chlorine (e.g., hydrogen chloride or silicontetrachloride) may be introduced into the system shown in FIG. 1 in oneor more make-up streams to replace hydrogen and chlorine that exits thesystem as an impurity in any of the product streams or impurity purgestreams (not shown). These make-up streams may supply hydrogen and/orchlorine to the system (or other compounds which contain hydrogen and/orchlorine atoms) at any number of process points including, for example,addition of hydrogen to the hydrogenation reactor 60 or as a carrier gasto the fluidized bed reactor 30 or addition of hydrogen chloride to thechlorination reactor 7. In some embodiments of the present disclosure,the ratio of hydrogen chloride added as a make-up to the amount ofhydrogen chloride circulating in the substantially closed-loop system isless than about 1:10, less than about 1:20, less than about 1:50 or evenless than about 1:100 (e.g., from about 1:250 to about 1:10 or fromabout 1:100 to about 1:20). In addition or alternatively, the ratio ofhydrogen (i.e., H₂ gas) added as a make-up to the amount of hydrogencirculating in the substantially closed-loop system (i.e., the amount ofhydrogen gas, H₂ and not hydrogen included within other molecules) isless than about 1:10, less than about 1:20, less than about 1:50 or evenless than about 1:100 (e.g., from about 1:250 to about 1:10 or fromabout 1:100 to about 1:20).

Hydrogen and/or chlorine make-up may also be characterized by the molarratio of these gases added as a make-up to polycrystalline product thatis produced. In several embodiments of the present disclosure, the molarratio of chlorine (i.e., based on the moles of chlorine atoms (Cl))added as a make-up, including chlorine gas itself (if any) and chlorineatoms that form part of chlorine-containing compounds (e.g., HCl, SiHCl₃and/or SiCl₄) to polycrystalline silicon product that is produced (notincluding silicon dust) is less than about 2:1 and, as in otherembodiments, less than about 1:1, less than about 1:1.2, less than about1:1.5, less than about 1:2 or less than about 1:2.5 (e.g., from about2:1 to 1:5 or from about 1:1 to about 1:5). The molar ratio of hydrogen(i.e., based on the moles of hydrogen atoms (H)) added as a make-up,including hydrogen gas itself (if any) and hydrogen atoms that form partof other hydrogen-containing compounds (e.g., HCl, SiHCl₃, SiCl₄ and/orSiH₄ but excluding hydrogen included within water that is used toseparate hydrogen from hydrogen chloride in a bubbler-type system) topolycrystalline silicon product that is produced may be less than about1:1 and, as in other embodiments, less than about 1:2, less than about1:3, less than about 1:5, less than about 1:10 (e.g., from about 1:1 to1:20 or from about 1:2 to about 1:10). In some embodiments, no hydrogenis added to the process as a make-up stream. Additionally notrichlorosilane, silicon tetrachloride or silane is typically added tothe system; rather, these compounds are produced and consumed within thesystem itself.

It is to be noted that, unless otherwise stated, the variousconcentrations, concentration ranges, percent inclusions, ratios,operating parameters (e.g., temperatures, pressures, conversion) and thelike recited herein, are provided for illustration purposes only andtherefore should not be viewed in a limiting sense. It is to beadditionally noted that all various combinations and permutations ofcompositions, concentrations, percent inclusions ratios, components,operating parameters and the like are intended to be within the scope ofand supported by the present disclosure.

Closed-Loop Systems for Producing Polycrystalline Silicon

The processes described above may be incorporated into a substantiallyclosed-loop system for producing polycrystalline silicon. Such systemsabove may be substantially closed-loop with respect to trichlorosilane,silicon tetrachloride, silane, hydrogen chloride and/or hydrogen. Inseveral embodiments of the present disclosure and as shown in FIG. 1,the system includes a chlorination reactor 7 in which hydrogen chlorideis contacted with silicon to produce trichlorosilane and silicontetrachloride. The system includes a disproportionation system 76 inwhich trichlorosilane is converted to silane or dichlorosilane and afluidized bed reactor 30 in which silane or dichlorosilane is decomposedto produce polycrystalline silicon 27. The system includes ahydrogenation reactor 60 in which silicon tetrachloride and hydrogen areintroduced to produce trichlorosilane. The system may include a strippercolumn (not shown) to remove light end impurities prior to introductionof silicon tetrachloride and trichlorosilane 26 into thedisproportionation system 76.

Referring now to FIG. 2, the disproportionation system 76 includes afirst distillation column 65 for separating silicon tetrachloride into abottoms fraction 57 and to separate dichlorosilane and trichlorosilaneinto an overhead fraction 69. A second distillation column 67 separatestrichlorosilane into a bottoms fraction 5 and dichlorosilane into anoverhead fraction 15. A first disproportionation reactor 50 produces afirst disproportionation reactor product gas 9 that containsdichlorosilane and silicon tetrachloride. A second disproportionationreactor 52 produces a second disproportionation reactor product gas 98that contains silane and trichlorosilane. A third distillation column 56separates silane into an overhead fraction 29 and trichlorosilane into abottoms fraction 94.

The system may include various conveying apparatus for transferringvarious components within the system. The system may include a conveyingapparatus to convey trichlorosilane from the hydrogenation reactor 60 tothe disproportionation system 76. A conveying apparatus conveys silaneor dichlorosilane from the disproportionation system 76 to the fluidizedbed reactor 30 and a conveying apparatus conveys trichlorosilane fromthe chlorination reactor 7 to the disproportionation system 76. Thesystem may also include a conveying apparatus for conveying silicontetrachloride from the disproportionation system 76 to the hydrogenationreactor 60.

The system may include a separation system 4 to separate hydrogen,hydrogen chloride, silicon tetrachloride and trichlorosilane. Aconveying apparatus conveys trichlorosilane and silicon tetrachloridefrom the separation system 4 to the disproportionation system 76 and aconveying apparatus conveys hydrogen chloride from the separation system4 to the chlorination reactor 7. A conveying apparatus conveys hydrogenfrom the separation system 4 to the hydrogenation reactor 60 and aconveying apparatus transfers hydrogenated gas from the hydrogenationreactor 60 to the separation system 4. Another conveying apparatusconveys chlorinated gas from the chlorination reactor 7 to theseparation system 4.

Referring now to FIG. 3, the separation system 4 may include achlorosilane separator 40 for separating trichlorosilane and silicontetrachloride 26 from hydrogen and hydrogen chloride 42 and a hydrogenseparator 47 for separating hydrogen 22 from hydrogen chloride 6. Asdiscussed above, the chlorosilane separator may be a vapor-liquidseparator (e.g., knock-out drum) and the hydrogen separator may be avapor-liquid separator or a bubbler.

In this regard, suitable conveying apparatus for use in the systems ofFIGS. 1-3 are conventional and well known in the art. Suitable conveyingapparatus for the transfer of gases include, for example, recirculationfans, compressors or blowers. Suitable conveying apparatus for transferof liquids include, for example, pumps and compressors and suitableconveying apparatus for transfer of solids include, for example, drag,screw, belt and pneumatic conveyors. In this regard, it should beunderstood that use of the phrase “conveying apparatus” herein is notmeant to imply direct transfer from one unit of the system to anotherbut rather only that the material is transferred from one unit toanother by any number of indirect transfer parts and/or mechanisms. Forinstance, material from one unit may be conveyed to further processingunits (e.g., purification or storage units used to provide a bufferbetween continuous or batch-wise processes) and then conveyed to thesecond unit. In this example, each unit of conveyance including theintermediate processing equipment itself may be considered to be the“conveying apparatus” and the phrase “conveying apparatus” should not beconsidered in a limiting sense.

All equipment used in the systems for producing polycrystalline siliconmay be resistant to corrosion in an environment that includes exposureto compounds used and produced within the system. Suitable materials ofconstruction are conventional and well-known in the field of thisdisclosure and include, for example, carbon steel, stainless steel,MONEL alloys, INCONEL alloys, HASTELLOY alloys, nickel and non-metallicmaterials such as quartz (i.e., glass), and fluorinated polymers such asTEFLON, KEL-F, VITON, KALREZ and AFLAS.

As shown in FIG. 1, the systems and processes described herein aresubstantially closed loop with respect to trichlorosilane, silicontetrachloride, silane, hydrogen chloride and/or hydrogen in that thesystem does not include trichlorosilane, silicon tetrachloride, silane,hydrogen, hydrogen chloride or trichlorosilane in the inlet stream 3 andthese compounds are not removed from the system in the outlet stream 27.In this regard, it should be understood that amounts of trichlorosilane,silicon tetrachloride, silane, hydrogen chloride and/or hydrogen may beremoved from the system in a purge stream and may be fed into the systemor process as in a make-up stream. Make-up of these compounds may beachieved by addition of the compounds to any process stream as may bedetermined by those of skill in the art.

It should be understood that the processes and systems described abovemay include more than one of any of the recited units (e.g., reactors,columns and/or separation units) and that multiple units may be operatedin series and/or in parallel without departing from the scope of thepresent disclosure. Further in this regard, it should be understood thatthe process and systems that are described are exemplary and theprocesses and systems may include additional units which carry outadditional functions without limitation.

When introducing elements of the present disclosure or the variousembodiments thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above apparatus and methodswithout departing from the scope of the disclosure, it is intended thatall matter contained in the above description and shown in theaccompanying figures shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A substantially closed-loop process for producingpolycrystalline silicon, the process comprising: a) introducing anamount of hydrogen chloride and silicon into a chlorination reactor toproduce a chlorinated gas comprising trichlorosilane, silicontetrachloride and hydrogen; b) separating hydrogen from the chlorinatedgas produced from the chlorination reactor to obtain a remaining streamcomprising trichlorosilane and silicon tetrachloride; c) introducing theremaining stream comprising trichlorosilane and silicon tetrachloride toa disproportionation system to produce silicon tetrachloride anddichlorosilane; wherein the disproportionation system comprises a firstdistillation column, a second distillation column and adisproportionation reactor, wherein: the remaining stream comprisingtrichlorosilane and silicon tetrachloride and a disproportionationreactor product stream are introduced into the first distillation columnto separate silicon tetrachloride into a bottoms fraction and toseparate dichlorosilane and trichlorosilane into an overhead fraction;introducing the overhead fraction produced from the first distillationcolumn into the second distillation column to separate trichlorosilaneinto a bottoms fraction and dichlorosilane into an overhead fraction;and introducing the bottoms fraction produced from the seconddistillation column into the disproportionation reactor to produce thedisproportionation reactor product stream comprising dichlorosilane andsilicon tetrachloride; d) introducing (1) dichlorosilane from the seconddistillation column of the disproportionation system and (2) hydrogen asa carrier gas into a fluidized bed reactor to produce polycrystallinesilicon and an effluent gas comprising hydrogen and unreacteddichlorosilane, the fluidized bed reactor being separate from thedisproportionation system, wherein an amount of hydrogen separated fromthe chlorinated gas is used as the carrier gas; e) introducing an amountof silicon tetrachloride from the first distillation column of thedisproportionation system and an amount of hydrogen from the effluentgas into a hydrogenation reactor to produce a hydrogenated streamcomprising trichlorosilane and hydrogen chloride; and f) separatinghydrogen chloride from the hydrogenated stream and returning an amountof the separated hydrogen chloride to the chlorination reactor in stepa).
 2. The process as set forth in claim 1 wherein the hydrogenatedstream further comprises unreacted hydrogen and unreacted silicontetrachloride, the hydrogenated stream being introduced into aseparation system to separate the remaining stream comprisingtrichlorosilane and unreacted silicon tetrachloride from hydrogen andunreacted hydrogen chloride.
 3. The process as set forth in claim 2wherein the separation system comprises: a chlorosilane separator forseparating the remaining stream comprising trichlorosilane and silicontetrachloride from hydrogen and hydrogen chloride; and a hydrogenseparator for separating hydrogen from hydrogen chloride.
 4. The processas set forth in claim 3 wherein the chlorosilane separator is avapor-liquid separator.
 5. The process as set forth in claim 3 whereinthe hydrogen separator is a vapor-liquid separator or a bubbler.
 6. Theprocess as set forth in claim 2 wherein the chlorinated gas dischargedfrom the chlorination reactor further comprises unreacted hydrogenchloride and wherein the chlorinated gas is introduced into theseparation system.
 7. The process as set forth in claim 1 wherein theremaining stream of step b) is introduced into a stripper column toremove light end impurities prior to introduction into thedisproportionation system.
 8. The process as set forth in claim 1comprising adding chlorine as a make-up, wherein the molar ratio ofhydrogen chloride added as a make-up to the amount of hydrogen chloridecirculating within the substantially closed-loop process is less thanabout 1:10.
 9. The process as set forth in claim 1 comprising addinghydrogen as a make-up, wherein the molar ratio of hydrogen gas added asa make-up to the amount of hydrogen circulating in the substantiallyclosed-loop process is less than about 1:10.
 10. The process as setforth in claim 1 comprising adding chlorine as a make-up, wherein themolar ratio of chlorine added as a make-up to polycrystalline siliconproduct that is produced is less than about 2:1.
 11. The process as setforth in claim 1 comprising adding hydrogen as a make-up, wherein themolar ratio of hydrogen added as a make-up to polycrystalline siliconproduct that is produced is less than about 1:1.
 12. The process as setforth in claim 1 wherein the first distillation column and thedisproportionation reactor are separate units.
 13. The process as setforth in claim 1 wherein the effluent gas from the fluidized bed reactorcomprises hydrogen chloride, the process further comprising: separatingthe hydrogen chloride from the effluent gas; and introducing an amountof separated hydrogen chloride from the effluent gas into thechlorination reactor to produce the chlorinated gas.
 14. The process asset forth in claim 1 wherein an amount of hydrogen separated from thechlorinated gas is introduced into the hydrogenation reactor to producetrichlorosilane.
 15. The process as set forth in claim 1 whereinhydrogen is directed toward a reactor wall of the fluidized bed reactor.16. The process as set forth in claim 1 wherein hydrogen is added to thefluidized bed reactor as a make-up stream for hydrogen.